Silicon wafers are commonly marked with serial numbers, bar codes, or other indicia for identifying and tracking wafers throughout the process steps of semiconductor device fabrication. Typically, the indicia are created by forming small surface disruptions in an otherwise uniformly flat and reflective (mirror-like or specular) surface of the wafer. These surface disruptions can be formed using a high intensity laser light, or a diamond scribe, for example, to selectively remove or redistribute the material that forms the reflective surface of the wafer, thereby producing small pits in the surface having, for example, a cup-shaped cross-section.
To access the identification and tracking information represented by the indicia, a machine vision system can be used to acquire and analyze an image of the indicia. Image acquisition includes the step of image formation, i.e., forming an image of the indicia on, for example, an image sensor, such as a CCD array in a CCD camera. The CCD array then provides a signal representing the indicia that is digitally processed to determine the information represented by the indicia.
In practice, the step of forming an image of the indicia can be complicated by the presence of irregularities on the wafer surface, or by changes in surface characteristics resulting from the various deposition steps involved in the semiconductor manufacturing process. In fact, known image formation schemes do not provide adequate performance in certain important situations wherein the indicia have been degraded by process steps, resulting in indicia reading errors. Moreover, existing imaging systems are experiencing further difficulties detecting the newer "soft marks". To form a readable image of indicia on the surface of a semiconductor wafer throughout a series of semiconductor process steps, the readability of the output image produced by the image formation system must be unaffected by the consequent changes in the appearance of the indicia.
Known systems for detecting indicia formed in the surface of a semiconductor wafer include a light source for illuminating the surface of the wafer, and a camera that has a lens system with an adjustable circular aperture stop for forming images on an imaging device, such as a CCD array. In such systems, some fraction of the light rays transmitted towards the portion of the wafer surface bearing the indicia are reflected by the wafer surface, and are then received by the camera to form an image of the indicia.
To adapt to the changes in the appearance of the indicia due to the sequence of processing steps, it is known to condition, modulate, shape or otherwise control the transmitted light, i.e., the light rays transmitted towards the portion of the wafer surface bearing the indicia.
For example, it is known to selectively block a portion of the transmitted light, i.e., the set of all light rays emitted by the light source, before the transmitted light impinges upon the wafer surface. In particular, Wilt et al., U. S. Pat. Nos. 5,231,536, and 5,469,294 teach an illumination system for recognition of indicia on a reflective substrate having a plurality of light sources, and a plurality of opaque masks and/or baffles that stop selected light rays emitted by the light sources from reaching the reflective substrate.
Other methods of controlling transmitted light before it reaches the surface of a mirror-like substrate include collimating the light, such that only light rays characterized by substantially similar angle of incidence impinge upon the surface of the substrate. Light can be substantially collimated by placing the light source at a great distance from the surface of the substrate, or by use of light collimating optics. Also, controlling the angle of incidence of the transmitted light in other ways can be employed.
However, these and other known systems for imaging indicia on a mirror-like substrate tend to be large, expensive to manufacture, and difficult to integrate into existing equipment, such as wafer probers, and other semiconductor device manufacturing equipment. They also tend to be too heavy to mount on existing robot arm assemblies.